Olefin polymerization catalyst systems can be classified into Ziegler-Natta and metallocene catalyst systems, and these highly active catalyst systems have been developed in compliance with their characteristics. Ziegler-Natta catalyst has been widely applied to existing commercial processes since it was developed in the 1950's. However, since the Ziegler-Natta catalyst is a multi-active site catalyst in which a plurality of active sites are mixed, it has a feature that molecular weight distribution is broad. Also, since a compositional distribution of comonomers is not uniform, there is a problem that it has a limitation to secure the desired physical properties.
Meanwhile, the metallocence catalyst includes a main catalyst whose main component is a transition metal compound, and an organometallic compound cocatalyst whose main component is aluminium. Such a catalyst is a single-site catalyst which is a homogeneous complex catalyst, and offers a polymer having a narrow molecular weight distribution and an uniform composition distribution of comonomers, depending on the single site characteristics. The stereoregularity, copolymerizing properties, molecular weight, crystallinity and the like of the resulting polymer can be controlled by changing the ligand structure of the catalyst and the polymerization condition.
U.S. Pat. No. 5,914,289 discloses a method of controlling the molecular weight and the molecular weight distribution of polymers using metallocene catalysts which are respectively supported on supports. However, a large amount of solvent and a long period of time are required to prepare the supported catalysts, and the process of supporting metallocene catalysts on the respective support is troublesome.
Korean Patent Application No. 10-2003-0012308 discloses a method of controlling the molecular weight distribution of polymers by changing and polymerizing a combination of catalysts in a reactor by supporting a bi-nuclear metallocene catalyst and a mononuclear metallocene catalyst on a support with an activator. However, this method has a limitation to achieve the properties of the respective catalysts simultaneously. In addition, there is a disadvantage that a metallocene catalyst portion is departed from a supported component of the catalyst to cause fouling in the reactor.
Therefore, in order to solve the above-mentioned disadvantages, there is a need to develop a method for preparing olefin based polymers with the desired physical properties by easily preparing a supported metallocene catalyst having an excellent activity.
On the other hand, a linear low density polyethylene is prepared by copolymerizing ethylene and alpha-olefins using a polymerization catalyst under a low pressure. Thus, this is a resin having a narrow molecular weight distribution which has a certain length of a short chain branch, without a long chain branch. The linear low density polyethylene film has, in addition to the characteristics of a typical polyethylene, high strength and elongation at break, and exhibits excellent tear strength and falling weight impact strength. The use of the linear low density polyethylene film increases in the stretch film, overlapping film or the like which is difficult to apply to existing low density polyethylene or high density polyethylene.
However, the linear low density polyethylene using 1-butene or 1-hexene as a comonomer is mostly prepared in a single gas phase reactor or a single loop slurry reactor, and the productivity is high as compared to a process using 1-octene comonomer, but these products also have limits in the catalyst technology and process technology. Thus, they are problematic in that their physical properties are greatly inferior to when using 1-octene comonomer and the molecular weight distribution is narrow and thus the processability is poor.
Many studies are conducted to improve these problems, and U.S. Pat. No. 4,935,474 discloses a process for producing polyethylenes having a broad molecular weight distribution by using two or more metallocene compounds. U.S. Pat. No. 6,828,394 discloses a process for producing polyethylenes having an excellent processability and particularly suitable for the film. In addition, U.S. Pat. No. 6,841,631 and U.S. Pat. No. 6,894,128 disclose that polyethylenes having a bimodal or multimodal molecular weight distribution is prepared by a metallocene catalyst comprising at least two metal compounds and thus it can be used for a film, a blow molding, a pipe and the like. However, there are still problems that although the processability of these products was improved, the dispersion state per molecular weight within the unit particle is not uniform and thus the extruded appearance is rough even under a relatively good extrusion condition, and the physical property is not stable.
Given the above circumstances, the preparation of superior products, having a balance between the physical property and the processability is continuously required, and further improvement for this is necessary.